Let's learn about transition metals having ions of different colours with their variable oxidation states in A-Level Chemistry!
Reaction of Iron
Iron (II) can be oxidised to Iron (III) in acid solution via titration.
Iron (II) is oxidised to Iron (III) when it reacts with manganate (VII) ions MnO4-:
Manganese is reduced from +7 to +2:
MnO4- + 5e- + 8H+ --> Mn2+ + 4H2O
Iron is oxidised from +2 to +3:
Fe2+ --> Fe3+ + e-
![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4979856/432045_592132.jpeg)
The same thing happens when iron (III) reacts with dichromate (VI) ions Cr2O72-:
Chromium is reduced from +6 to +3:
Cr2O72- + 6e- + 14H+ --> 2Cr3+ + 7H2O
Iron is oxidised from +2 to +3:
Fe2+ --> Fe3+ + e-
![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4979856/394343_438221.jpeg)
Reaction of Chromium
In aqueous solution, Cr(VI) exists either as a chromate(VI) ion or a dichromate(VI) ion depending on the acidicty of solution. The equilibrium is established below:
![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4979856/171197_739120.jpeg)
![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4979856/945022_789960.png)
The dichromate(VI) is more stable in acidic conditions and the chromate(VI) in alkaline conditions.
IN acid solutions, Cr3+and Cr2+ ions can be formed with zinc as the reducing agent:
Chromium is reduced from +6 to +3:
Cr2O72- + 6e- + 14H+ --> 2Cr3+ + 7H2O
Zinc is oxidised from 0 to +2:
Zn --> Zn2+ + 2e-
![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4979856/342993_289651.jpeg)
Then there is further reduction for chromium from 3+ to 2+:
![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4979856/185776_623217.jpeg)
In alkaline solution, Cr3+ can be oxidised to CrO42- using H2O2 and NaOH:
![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4979856/77361_920440.jpeg)
Reaction of Cobalt
Cobalt (II) can be oxidised to cobalt (III) using H2O2 in alkaline solution:
![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4979856/207533_504384.jpeg)
In ammoniacal solution, Co2+ ions can also be oxidised to Co3+ ions when left in air:
![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4979856/750202_238647.jpeg)
Cobalt is oxidised from +2 to +3:
[Co(NH3)6]2+ --> [Co(NH3)6]3+ + e-
Oxygen is reduced from 0 to -2:
O2 + 2H2O --> [Co(NH3)6]3+ +4OH-
4 [Co(NH3)6]2+ +O2 + 2H2O --> [Co(NH3)6]3+ +4OH-
![broken image](http://custom-images.strikinglycdn.com/res/hrscywv4p/image/upload/c_limit,fl_lossy,h_9000,w_1200,f_auto,q_auto/4979856/120317_107612.png)
Drafted by Eunice (Chemistry)